Iodine Oxidative Addition to Isomeric Platinum(II) Phosphine Complexes

Solvent-Dependent Reactivity and Photochemistry of Dinuclear and Mononuclear Platinum(IV) Azido Triazaolato Complexes

Reaction between the platinum(IV) azido complex trans,trans,trans-[Pt(py)2(N3)2(OH)2] (1) and 1,4-diphenyl-2-butyne-1,4-dione 2 in MeCN produces the intermediate peroxide-bridged dimeric platinum(IV) azido triazolato species (5), which has been characterised by X-ray crystallography. However, if the reaction between 1 and 2 is conducted in MeOH it results in decomposition. Over time in MeCN, dimer (5) converts into mononuclear complexes trans,trans,trans-[Pt(py)2(N3)(triazole)(OH)2] (3 a/3 b), which are in dynamic exchange. If resuspended in protic solvents (MeOH,H2O), 3 a/3 b undergo a slow (22 d) irreversible rearrangement to a cyclised platinum(IV) species 4 which contains a formally N,O-chelated ligand. Conversion of 3 a/3 b to 4 in d 4-MeOH can be accelerated (384x) by irradiation with visible light, although continued irradiation also produces N3 . and OH. radicals, and the [4-N3]+ species can be readily detected by ESI-MS. Solvent choice significantly effects both the cycloaddition reaction between 1 and 2, and the stability of the resultant complexes.

Shedding Light on the Nature of Photoinduced States Formed in a Hydrogen-Generating Supramolecular RuPt Photocatalyst by Ultrafast Spectroscopy

Photoinduced electronic and structural changes of a hydrogen-generating supramolecular RuPt photocatalyst are studied by a combination of time-resolved photoluminescence, optical transient absorption, and X-ray absorption spectroscopy. This work uses the element specificity of X-ray techniques to focus on the interplay between the photophysical and -chemical processes and the associated time scales at the catalytic Pt moiety. We observe very fast (<30 ps) photoreduction of the Pt catalytic site, followed by an ∼600 ps step into a strongly oxidized Pt center. The latter process is likely induced by oxidative addition of reactive iodine species. The oxidized Pt species is long-lived and fully recovers to the original ground state complex on a >10 μs time scale. However, the photosensitizing Ru moiety is fully restored on a much shorter ∼300 ns time scale. This reaction scheme implies that we may withdraw two electrons from a catalyst that is activated by a single photon.

Electrophilic halogenation-reductive elimination chemistry of organopalladium and -platinum complexes

CONSPECTUS: Transition metal-catalyzed organic transformations often reveal competing reaction pathways. Determining the factors that control the selectivity of such reactions is of extreme importance for the design of reliable synthetic protocols. Herein, we present the account of our studies over the past decade aimed at understanding the selectivity of reductive elimination chemistry of organotransition metal complexes under electrophilic halogenation conditions. Much of our effort has focused on finding the conditions for selective formation of carbon (aryl)-halogen bonds in the presence of competing C-C reductive elimination alternatives. In most cases, the latter was the thermodynamically preferred pathway; however, we found that the reactions could be diverted toward the formation of aryl-iodine and aryl-bromine bonds under kinetic conditions. Of particular importance was to maintain the complex geometry that prohibits C-C elimination while allowing for the elimination of carbon-halogen bonds. This was achieved by employing sterically rigid diphosphine ligands which prevented isomerization within a series of Pt(IV) complexes. It was also important to understand that the neutral M(IV) products often observed or isolated in the oxidative addition reactions are not necessarily the intermediates in the reductive elimination chemistry as it generally takes place from unsaturated species formed en route to relatively stable M(IV) complexes. While aryl-halide reductive elimination for heavier halogens can be competitive with aryl-aryl coupling in diaryl M(IV) complexes, the latter reaction always prevails over aryl-fluoride bond formation. Even when one of the aryl groups is a part of a rigid cyclometalated ligand C-C coupling is still the dominant reaction pathway. However, when one of the aryl groups is replaced with a phenolate donor aryl-F bond formation becomes preferred over C-O bond elimination. During our studies, other interesting reactions have been discovered. For example, the fluorination of the C(sp(3))-H bond can be very selective and compete favorably with C-C coupling. Also, in electron-poor complexes, metal oxidation can have higher energy than oxidation of the coordinated iodo ligand resulting in I-F elimination instead of the formation of aryl-I bond. Overall, electrophilic fluorination can lead to often very selective elimination reactions giving new C-C, C-I, C-F, or I-F bonds, with this selectivity dependent on the metal center, supporting ligands, complex geometry, and electrophilic fluorine source. Together with the many reports on the halogenation of organometallic compounds that appeared in recent years, our results contribute to understanding the requirements for selective transformations under electrophilic conditions and design of new synthetic methods for making organohalogen compounds.

Pd(II)-catalyzed C-H iodination using molecular I2 as the sole oxidant

Pd-catalyzed ortho-C-H iodination directed by a weakly coordinating amide auxiliary using I2 as the sole oxidant was developed. This reaction is compatible with a wide range of heterocycles including pyridines, imidazoles, oxazoles, thiazoles, isoxazoles, and pyrazoles.

Aryl-Halide versus Aryl−Aryl Reductive Elimination in Pt(IV)−Phosphine Complexes

Upon the addition of Br2 to complexes (P-P)Pt(Ar)2, two different products were observed, depending on the bite angle of the bidentate phosphine ligand: a Pt(II) aryl bromide complex, the product of C-Br reductive elimination, and Pt(IV) oxidative addition complex. At high temperatures, the latter exclusively gave the product of the C-C reductive elimination.

Platinum Oxide Catalyzed Silylation of Aryl Halides with Triethylsilane: An Efficient Synthetic Route to Functionalized Aryltriethylsilanes

The first platinum-catalyzed selective silylation of aryl halides including aryl iodides and bromides having an electron-withdrawing group is described. The reaction takes place rapidly in NMP with triethylsilane as a silicon source and sodium acetate to provide functionalized aryltriethylsilanes in moderate to good yields. Heteroaromatic halides also were found to be readily silylated with triethylsilane. The procedure is chemoselective and tolerates a wide variety of functional groups.